Electrochemical machining and micromachining
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Electrochemical Machining and Micromachining. Summer school on electrochemical engineering, Palic, Republic of Serbia Prof. a.D. Dr. Hartmut Wendt, TUD. The fundamentals of electrochemical surface treatment. Electrochemical surface treatment is based on anodic metal dissolution.

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Electrochemical machining and micromachining

Electrochemical Machining and Micromachining

Summer school on electrochemical engineering, Palic, Republic of Serbia

Prof. a.D. Dr. Hartmut Wendt, TUD


The fundamentals of electrochemical surface treatment
The fundamentals of electrochemical surface treatment

  • Electrochemical surface treatment is based on anodic metal dissolution.

  • Metal dissolution by a) active dissolution, b)transpassive dissolution

  • Dimensional resolution is mainly determined by current and potential distribution around a cathodic matrix

  • Forced convection removes bubbles (by H2 and O2 evolution) and oxidic and hydroxidic debris e.g. Fe(OH)3 and other oxidation-solvolysis products



Electrochemical machining and micromachining

Electrochemical shaping of metals metal dissolution

Active dissol. and mass transfer

Transpassive dissol. With fast sweep

Transpassive dissolution with mechanical scraping


Electrochemical machining and micromachining

Some examples of electrochemical machining of hard metals: metal dissolution Primary Current density distribution


Current density distributions
Current density distributions metal dissolution

  • Primary: neglects charge transfer kinetics and influence of mass transfer.Decisive: only distributions of pure Ohmic resistances

  • Secondary: Adding charge transfer resistances to purely Ohmic resistances

  • Tertiary: Mainly determined by mass transfer conditions



Electrochemical machining and micromachining

Addition of electrolyte resistances R plates and at the electrode edgep and Rv add to charge transfer resistance to give primary and secondary current distributions


Even current density distribution

Even current density distribution plates and at the electrode edge

At rotating disc electrode under mass transport limited condition ( limiting current density) is a typical tertiary c.d. distribution


Electropolishing

Electropolishing plates and at the electrode edge

Mass transfer controlled transport of dissolution products through a thin, statistically fluctuating layer of debris generates the polishing effect

Current densities amount from hundred to several hundred mA cm-2


Electropolishing electrolytes
Electropolishing electrolytes plates and at the electrode edge

  • Composition given in lecture manuscript

  • Almost all contain phosphoric acid

  • Almost all – exception electropolishing W – are strongly acidic

  • Some contain organic cosolvents

  • Are obtained and optimized by trial and error


Electrochemical machining electrolytes
Electrochemical machining electrolytes plates and at the electrode edge

  • Are neutral (neither basic nor acidic) with the exception of basic electrolyte for molybdenum

  • Most of them contain sodium nitrates or perchlorate

  • Current densities amount to several A cm-2

  • Copious exchange of electrolyte must be secured to remove Joule`s heat and all debris


Electrochemical micromachining

Electrochemical micromachining plates and at the electrode edge




Electrochemical machining and micromachining

ح more slowly than next to= l x ρ x Cspec

with l = length of current line

ρ = specific resistance of electrolyte (approximately 10 Ω cm)

ζ is charging time; as potential changes exponentially with time:

Φo – Φ = (Φo – Φ )t=(1-exp(-t/ ح)


Improve the resolution of anodic dissolution from millimetres to micrometres

Improve the resolution of anodic dissolution from millimetres to micrometres

Applying pulses in nanoseconds instead of direct currents


Electrochemical machining and micromachining

Example from L.Cagnon, V. Kirchner, M. Kock, millimetres to micrometresR. Schuster, G. Ertl, Th. Gmelin and H. Kueck, Z. Phys. Chem. 217, (2003), 299 - 313


Electrochemical machining and micromachining

Example from M. Kock, V. Kirchner and millimetres to micrometresR. Schuster, Electrochim. Acta 48, (2003) 3213 - 3219


Electrochemical machining and micromachining
The LIGA – Process for building millimetres to micrometresMicro-structures by x-ray-assisted masking and cathodic metal deposition

  • Resolution is determined by precision of masks and their copy on photo-resist – hence x-ray copying


Summary
Summary millimetres to micrometres

  • With maximal cutting rates corresponding to several A cm-2 electrochemical machining is too slow to be generally applicable instead of mechanical machining

  • But ultrahard alloys can only be treated by electrochemical machining which usually gives also a good polishing finish

  • Applying nanosecond pulses increases the dimensional resolution, so that also micrometer structures can be produced – it is still an open question how to utilize these possibilities in commercial processes.